Polyesters have been replacing glass and metal packaging materials due to their lighter weight, decreased breakage compared to glass, and potentially lower cost. One major deficiency with standard polyesters, however, is its relatively high gas permeability. This curtails the shelf life of carbonated soft thinks and oxygen sensitive beverages or foodstuffs such as beer, wine, tea, fruit juice, ketchup, cheese and the like. Organic oxygen scavenging materials have been developed partly in response to the food industry's goal of having longer shelf-life for packaged food. These oxygen scavenging materials are incorporated into at least a portion of the package and remove oxygen from the enclosed package volume which surrounds the product or which may leak into the package, thereby inhibiting spoilage and prolonging freshness.
Suitable oxygen scavenging materials include oxidizable organic polymers which may react with ingressing oxygen. One example of an oxidizable organic polymer is a polyether. The polyether is typically used as a polyester-ether copolymer and in low amounts of less than 10 weight percent of the packaging material. The polyester-ether is dispersed in the matrix polyester phase and interacts with a suitable oxygen scavenging catalyst that catalyzes the reaction of the ingressing oxygen with the polyether, Oxygen scavenging catalysts are typically transition metal compounds, for example an organic or inorganic salt of cobalt. Other examples include manganese, copper, chromium, zinc, iron and nickel.
To reduce the oxygen ingress into the filled contents of the container, small amounts of transition metal salts, such as cobalt salts, may be added to the resin composition. However, such additions, if not careful, may also impart coloration and haze to the container. Therefore, it is often desirable to minimize the amount of metal based oxygen scavenging catalysts.
Another problem with the addition of transition metal salts, cobalt salts in particular, is that the residual moisture content of the salts is sufficiently high. It is difficult to completely dry out these salts. The presence of water molecules is undesirable during a late addition (adding after the finisher) step or during a compounding step. It may be possible to add the transition metal salts, such as cobalt salts, at the beginning of the polycondensation reaction, wherein the molecular weight is still low and any moisture content could be handled during the process. However, in this case, the added transition metal, cobalt for example, precipitates which contribute to haziness during articles formation. Also, the added transition metal could catalyze the depolymerization reaction and other unwanted decomposition reactions.
It is a common industrial problem that the water content causes the an inferior viscosity characteristics, low intrinsic viscosity (IV) in particular. Low intrinsic viscosity translates to low molecular weight. The low-molecular weight polymer compositions are not effective in compounding resins for container manufacturing. Articles, bottles and containers prepared using low-molecular weight resins show inferior mechanical properties.
The present disclosure relates to solving this problem of low molecular weight resins by using a particular combination of alkali metal and alkaline earth metal salts with the cobalt salts used.
In the preparation of polymers by a compounding step, the intrinsic viscosity (IV) of the end product is of practical importance. There may be equipment limitations such as strand granulation due to a too low IV. It is desirable to achieve the end product IV as high as possible given process equipment limitations.
The present disclosure makes it possible to achieve the IV target of end product of at least 0.40 dL/g.